In the kraft paper pulping process, wood chips of coniferous trees are fed into a digester and a "white" cooking liquor containing sodium hydroxide and sodium sulfide is added. The contents are heated according to a predetermined schedule to complete the cooking reaction whereby resin acids and fatty acids are released from the wood chips and saponified. The resulting cooked pulp is separated from the residual cooking liquor, known as "black liquor". The sodium salts of the resin acids and fatty acids, commonly referred to as tall oil soap or "black liquor soap", are suspended in the black liquor. The tall oil soap is then recovered from the kraft black liquor by any one of various techniques known to those skilled in the art. The separated tall oil soap is treated with sulfuric acid to form crude tall oil. The main components of crude tall oil are fatty acids, rosin, and unsaponifiables. The crude tall oil can be further refined by vacuum fractionation to form tall oil fatty acids and tall oil-based rosin acids. The fatty acids and rosins are used in a variety of applications, such as the preparation of polyamides, rosin resins, adhesives and epoxy compositions.
In the processing of tall oil for the recovery of useful products, the oil is typically subjected to fractionation to separate various components. The fractional distillation of crude tall oil is accomplished in a multi-column configuration using high temperature and low partial pressures. In a first column, known as a "pitch stripper" or "depitching column", the fatty acids and rosin are removed together as a distillate stream with pitch as the bottoms product. A heads cut may also be taken in this first column. The distilled tall oil, a mixture of fatty acids and rosin, is fed to a fractionating column chain consisting of two or three columns in series where four fractions are generally taken: heads cuts; a pure, distilled tall oil fatty acids cut; a mixed fraction (25-30% rosin); and tall oil rosin.
Both the initial heads cut from the depitching column, and the subsequent heads cut from the fractionating columns, is referred to hereinafter collectively as "tall oil heads".
Tall oil heads are typically composed of 50-75% of a mixture of fatty acids comprising palmitic, oleic and linoleic acids. The remainder consists generally of unsaponifiable materials. The unsaponifiable material in tall oil include diterpene alcohols, aldehydes and hydrocarbons, sterols, fatty and wax alcohols, stilbenes, and other compounds. Palmitic acid is a major component of tall oil heads. Thus, heads tend to be soft solids at ambient temperature as the result of the high melting point (63.degree. C.) of palmitic acid. Tall oil heads are used as floatation agents that are used in agricultural emulsifiers, drilling fluids, soaps, and waterproofing agents, and as a source of palmitic acid. When admixed with distilled tall oil, tall oil heads can be used to make polyalkylene polyamide condensates that are used as corrosion inhibitors, as cationic flotation collectors and as asphalt emulsifiers.
Anthraquinone is used as a pulping catalyst in the pulping process. Anthraquinone has the effect of decreasing pulping times and improving yields of cellulosic materials from the pulping operation. Anthraquinone is typically introduced into pulping operations as a 50% water slurry, or as a neat powder. Much of the anthraquinone remains in the black liquor. As the black liquor is concentrated prior to burning in the kraft recovery boiler, the tall oil soap containing much of the anthraquinone separates. The tall oil soap is converted to tall oil by acidulation with, for example, sulfuric acid. Tall oil can contain as much as 1500 parts per million of anthraquinone or more. Anthraquinone concentrates in tall oil heads during distillation and can reach a concentration of 1.5 wt. % or more.
Anthraquinone is a solid at standard temperature and pressure, and sublimes from a solid to a gas at elevated temperature. During the fractionation of crude tall oil, anthraquinone collects and sublimes at the top of the fractionation tower. The more efficient the fractionation tower, the higher will be the concentration of anthraquinone at the tower top.
If the tall oil heads are cooled, the anthraquinone crystallizes, causing pluggage of coolers and associated equipment, which can force the entire distillation operation to be shut down for cleaning. Moreover, the high concentration of anthraquinone in tall oil heads may render the heads unsuitable for some purposes. Tall oil heads with a high anthraquinone content may cause anthraquinone contamination of the environment if used as a floatation collector additive.
U.S. Pat. No. 4,197,168 describes a fractional distillation process for removal of anthraquinone from tall oil heads. Pitch-stripped crude tall oil is distilled at reduced pressure (from about 25 torr at the top of the column) and a temperature of about 215.degree. C. to 290.degree. C. A heads cut taken from the top of the column contains about 80% of the anthraquinone admitted to the distillation column in the pitch-stripped crude tall oil. The anthraquinone-containing tall oil heads product contains from about 5% to 95% anthraquinone, 5% to 75% heads fatty acids, and 2.5% to 25% unsaponifiables. U.S. Pat. No. 4,197,168 states that the anthraquinone-containing tall oil heads fraction may be recycled to pulping operations or can be further rectified by further fractional distillation to provide a refined product further enriched in anthraquinone.
U.S. Pat. No. 4,197,168 purports to obtain tall oil heads cuts with at least 5% anthraquinone, up to as much as 95% anthraquinone. However, anthraquinone concentrations even as low as 1.4% in the heads fraction, and possibly lower depending on the distillation conditions, are problematic in that anthraquinone crystallizes upon cooling. The precipitated anthraquinone resists pumping and thereby upsets the reflux section of the distillation operation.
On the other hand, tall oil heads fractions with anthraquinone concentrations less than 10%, more typically less than 5%, can not serve as anthraquinone sources for pulping operations without further concentration, even though tall oil heads are otherwise compatible with pulping. (The heads fraction does not contain any components which would interfere with pulping.) Pulping operations utilize anthraquinone in the form of a 50% water slurry, added to the pulping mixture at a rate of about 0.65 pounds of slurry per ton of oven dried wood. Use of a tall oil heads cut containing 5% anthraquinone, or even 10% anthraquinone, as a pulping additive would leave the pulping mixture too dilute.
What is needed is an efficient method for removing anthraquinone from tall oil or tall oil fractions, particularly from tall oil heads. Removal of anthraquinone from tall oil heads avoids equipment plugging caused by anthraquinone solidification during tall oil heads processing. Removal of anthraquinone from tall oil heads also eliminates anthraquinone environmental contamination particularly where the heads are used as floatation agents which may be released into the environment. A process which not only removes anthraquinone, but also concentrates the same in a usable form, would be advantageous as a generator of anthraquinone for pulping operations.